1. Field of the Invention
The present invention relates to wireless communications; in particular, the present invention relates to multi-carrier modulation and demodulation techniques suitable for use in wireless healthcare, wireless body area networks, and wireless sensor networks.
2. Discussion of the Related Art
Multi-carrier modulation and demodulation techniques are used in broadband wireless communications. For example, orthogonal frequency division multiplexing (OFDM) has been extensively used. Early descriptions of OFDM may be found, for example, in (a) U.S. Pat. No. 3,488,445 (“Chang”), in “Orthogonal frequency multiplex data transmission system,” by R. W. Chang, which was filed on Nov. 14, 1966 and issued on Jan. 6, 1970; and (b) the article “Data transmission by frequency division multiplexing using the discrete Fourier transform” (“Weinstein”), by S. B. Weinstein and P. M. Ebert, published in IEEE Trans. Corn. Tech., vol. 19, no. 5, pp. 628-634, October 1971.
OFDM modulates information samples on a set of narrowband carriers at the transmitter. At the receiver, after analog-to-digital conversion (ADC), the OFDM signal may be demodulated using a fast Fourier transform (FFT). OFDM using narrowband carriers requires only relatively simple channel equalization. However, the high-speed ADC and the digital processing that follows incur high hardware cost and high power consumption. Such costs are not economical for simple, low cost, and low data rate products, such as those found in wireless healthcare applications.
The article “Slightly frequency-shifted reference ultra-wideband (UWB) radio” (“Goeckel”), by D. Goeckel and Q. Zhang, published in IEEE Trans. Commun., vol. 55, no. 3,: pp. 508-519, March 2007, discloses a dual-carrier differential modulation and demodulation scheme. Goeckel transmits one information symbol in each symbol period using a differential relationship between two carriers. At the receiver, the information symbol is demodulated and recovered using simple analog processing; Goeckel's system therefore avoids ADC and digital signal processing, thus significantly reducing system complexity and power consumption. However, because only two carriers are used, Goeckel's system takes advantage of very limited frequency resources. Goeckel's system therefore does not take advantage of frequency diversity inherent in multipath channels. Such frequency diversity represents significant efficiency in, for example, wireless body area network (WBAN) applications.
Complex multi-carrier modulation and demodulation techniques are used in a wide range of systems, such as the downlink of the long term evolution (LTE) system. Complex multi-carrier modulation and demodulation techniques in an LTE system is described, for example, in the article “Technical solutions for the 3G Long-Term Evolution” (“Ekström”), by H. Ekström et al., published in IEEE Commun. Mag., vol. 44, no. 3, March 2006, pp. 38-45. Real multi-carrier modulation and demodulation techniques are also used for wireless communication systems (e.g., the impulse radio). For example, the article “Impulse radio: how it works,” by M. Z. Win and R. A. Scholtz, IEEE Commun. Lett., vol. 2, no. 2, February 1998, pp. 36-38, discloses real multiple-carrier modulation and demodulation.